1. Field of the Invention
The present invention relates to an ink droplet ejection device, and more particularly to an ejection device for ejecting ink droplets utilizing deformation of a piezoelectric transducer.
2. Description of the Prior Art
A piezoelectric ink jet printing head has recently been proposed in the art. In the printing head known as a drop-on-demand type, ink contained in an ink channel is ejected in the form of droplet from an orifice when the volume of the ink channel is reduced resulting from deformation of a piezoelectric transducer, and the ink is supplemented into the ink channel from a valve opposite an orifice plate when the volume of the ink channel is increased. A multiplicity of such ejection units are closely juxtaposed so that a desired character or image is formed by ejecting ink droplets from selected ejection units.
An ink droplet ejection device of the type described above is disclosed in U.S. Pat. Nos. 4,992,808, 5,003,679 and 5,028,936. FIGS. 1 and 2 show an arrangement of such a conventional ejection device. As shown, an ejector array is made up of a piezoelectric ceramic wafer 1 and a cover plate 21. The piezoelectric ceramic wafer 1 has an inner surface formed with a plurality of ribs 2a, 2b, 2c, 2d extending in parallel to one another. Metal electrodes 11c, 11d are separately formed on the surface of the rib 2b. Likewise, metal electrodes 11e, 11f are also separately formed on the surface of the rib 2c. The piezoelectric wafer 1 has been polarized in the direction indicated by an arrow 51. The cover plate 21 is made of metal, glass or ceramic. The cover plate 21 is face-to-face bonded to the piezoelectric ceramic wafer 1 through an adhesive layer 12, thereby forming a plurality of ink channels 31a, 31b, 31c arranged in a horizontal direction. Each of the ink channels has a rectangular cross-section. The ribs defining the ink channel are deformable in a direction perpendicular to both the ink channel extending direction and the polarization direction, i.e., in the direction transversal to the direction in which the ink channels extend.
To eject an ink droplet from, for example, the ink channel 31b in accordance with print data, electric fields are applied between the metal electrodes 11c and 11d and between the metal electrodes 11e and 11f. Since the direction in which the electrical field is applied is orthogonal to the polarization direction of the piezoelectric ceramic wafer 1, the ribs 2b and 2c are deformed inwardly of the ink channel 31b pursuant to piezoelectric thickness shear effect. Due to the deformation of the ribs 2b and 2c, the volume of the ink channel 31b is reduced and thus the ink pressure is increased, causing to eject an ink droplet from an orifice 42 (see FIG. 2) formed on an orifice plate 41. When the application of the electric fields are stopped, the ribs 2b, 2c are restored to the original states, whereat ink is supplemented into the ink chamber 31b from an ink reservoir utilizing the fact that the ink pressure in the ink channel is reduced at the time of restoration of the ribs 2b, 2c.
Next, the manufacturing process of the ejector array will be described with reference to FIG. 2. The piezoelectric ceramic wafer 1 which has been polarized in the direction of arrow 51 is ground by a rotary diamond cutting disc to form U-shaped grooves 3 serving as ink channels. Metal electrodes are formed on the surfaces of the ribs by way of spattering. Thereafter, the cover plate 21 is bonded to the top faces 4a of the ribs. The orifice plate 41 is then bonded to the side face 4b of the piezoelectric ceramic wafer 1 so that the orifices 42 are in alignment with the ink channel positions.
The conventional ink droplet ejection array thus constructed is involved with a problem that a high driving voltage is required for ejecting the ink droplets. This is due to the fact that the deformation of the ribs is restrained by the cover plate bonded to one side of the transducer. Therefore, the ribs cannot be deformed as desired if it is driven with a low driving voltage.